Signal-processing method and active sonar implementing same

ABSTRACT

The invention relates to the field of underwater acoustics and more particularly to the field of signal processing in a low frequency (LF) active sonar system. 
     The present invention makes it possible to decrease the false alarm rate while retaining the classification of the objects. 
     The subject of the invention is a method of processing signals received corresponding to a signal emitted comprising by recurrence two pulses, a first Doppler tolerant broadband pulse of HFM type in particular and a second Doppler intolerant broadband pulse of BPSK type in particular, comprising:
         a step of detecting objects performed on the part of the signal received corresponding to the first pulses and providing an alarm for each object detected, and   a step of classifying the objects detected performed on the part of the signal received corresponding to the second pulses for the alarms satisfying at least one predetermined criterion.

FIELD OF THE INVENTION

The invention relates to the field of underwater acoustics and moreparticularly to the field of signal processing in a low frequency (LF)active sonar system.

BACKGROUND OF THE INVENTION

This type of system is generally towed from a surface vessel andcomprises a fish equipped with an LF emitter which tows a linearreceiving antenna furnished with acoustic or hydrophonic sensors. Such afish and such an emitter are for example described respectively inFrench Patents published under numbers 2735645 and 2776161. However, theinvention may be applied to all types of active sonars. It is well knownthat an active sonar emits recurrent acoustic pulses and that the echoesreceived in return are processed to detect and classify any targets.

When an active sonar operates in a zone such that the bottom isinsonified, the reverberation which originates from the bottom inessence greatly limits the operational usefulness of the sonar onaccount of the overly large number of false alarms which appear. This isparticularly true for shallow depths.

To reduce nuisance in a reverberating medium, it is known to useemission codes that harness the wide frequency bands, typically anoctave, of present-day transducers. These codes possess good distanceresolution, hence the large number of alarms that are produced.

It is known to emit at each recurrence, either an HFM (HyperbolicFrequency Modulation) code, or a BPSK (Binary Pulse Shift Keying) codeor an FP (Frequency Pulse) code.

The HFM code is Doppler tolerant: it therefore does not allowmeasurement of the Doppler induced by a target in motion but, on theother hand, the matched filtering on reception requires only a singlecopy.

The BPSK code is Doppler intolerant and is used to measure the Doppler;it allows the same detection performance as the HFM code but the matchedfiltering on reception requires a significant number of copies to carryout the matched filtering on reception, typically a number greater than200, and hence a correspondingly large processing cost.

OBJECT OF THE INVENTION

As for the FP code, it is used to measure the inherent Doppler of theemitter.

SUMMARY OF THE INVENTION

The present invention makes it possible to decrease the false alarm ratewhile retaining the classification of the objects.

The object of the invention is therefore a method of processing signalsreceived corresponding to a signal emitted comprising by recurrence twopulses, a first Doppler tolerant broadband pulse and a second Dopplerintolerant broadband pulse, comprising:

-   -   a step of detecting objects performed on the part of the signal        received corresponding to the first pulses and providing an        alarm for each object detected, and    -   a step of classifying the objects detected performed on the part        of the signal received corresponding to the second pulses for        the alarms satisfying at least one predetermined criterion.

At each recurrence, the two HFM and BPSK codes are emitted. Thedetection of the alarms is done with the HFM code and the estimation ofthe Doppler is done with the BPSK code on the alarms which exceed acertain threshold, so as to eliminate the bottom echoes. Statedotherwise:

-   -   detection with the HFM code    -   Doppler classification with the BPSK code.

Moreover, the bottom echoes being identified, the measurement of theinherent Doppler of the emitter is done by analyzing the bottom echoesproduced by the BPSK code.

DESCRIPTION OF THE FIGURES

The characteristics and advantages of the invention will become moreclearly apparent on reading the description, offered by way of example,and the figures pertaining thereto which represent:

-   -   FIG. 1, the successive steps of the method according to the        invention,    -   FIG. 2, the probability distributions of the measured Doppler        d_(m) for two hypotheses: H₀ for (stationary) bottom echo and H₁        for assumed true Doppler echo d_(i).

DETAILED DESCRIPTION

FIG. 1 represents the successive steps of the method according to theinvention.

In a known manner, the hydrophone signals undergo upstream processing(demodulation, filtering, amplification, etc.) and are then digitized.In the case of an active sonar, these signals contain the signalsemitted after propagation through the water via the direct path and thereflected paths to which are added the reverberated signals. Inparticular, among the signals reflected, the echoes originating from thesea bottom constitute a significant source of false alarms, inparticular at shallow depths.

According to the invention, at each recurrence are emitted two codedpulses, HFM and BPSK, whose characteristics make it possible to separatethem on reception. They can be emitted at different instants withtotally or partly overlapping frequency bands, or else be emittedsimultaneously in distinct frequency bands, or both at once.

Referring to FIG. 1, the processing of the hydrophone signals consistsfirstly in forming channels S1 in a known manner, this processing beingindependent of the code emitted.

To the signals of HFM channels is applied the matched filteringprocessing S2 consisting in correlating the signal received with a copyof the signal emitted which after rms detection provides signalsrepresentative of the energy as a function of channel (v) and of time(t) i.e. E_(HFM)(v,t).

The next step S3 consists in detecting and in sorting the alarms withregard to an energy criterion. In a conventional manner, the localmaxima are firstly searched for by comparison with a predeterminedthreshold. Thereafter, a normalization is performed by calculating foreach local maximum a value equal to (E_(HFM)−M)/σ where M is the mean ofthe reference noise, taken in the neighborhood of the “channels/time”space (v,t) and σ the corresponding standard deviation. Then, any maximaaround each maximum are eliminated if they have lower normed energy.Finally, the actual detection is obtained by comparing the noneliminatedmaxima with a normed energy threshold.

According to the invention, the matched filtering processing S5 on the“BPSK” channel signals is performed only on the alarms arising from theprocessing of the HFM pulses S4. The matched filtering processingcorresponding to the BPSK code which is Doppler tolerant requires thatthe channel signal be correlated with several Dopplerized copiescovering a range of given target velocities. Thus for an alarm areobtained as many signals as there are copies and form the Dopplerchannels.

The next step S6 consists in estimating the Doppler d and the associatedstandard deviation σ_(d) _(i) of the alarm “i” on the basis of thesignals arising from the Doppler channels. If d_(channel) is the Dopplergiven by the channel in which the alarm is to be found, the Doppler d isobtained by interpolation with the Dopplers of the adjacent channels.

The next step S7 consists in estimating the inherent Doppler d_(p) dueto the velocity of the antennas, emission and reception, with respect tothe bottom. It is estimated at each instant, either on the basis of aDoppler of the echoes originating from the bottom and detected by theBPSK code, or on the basis of the reverberation spectrum obtained by anFP code emitted with the HFM and BPSK codes. The standard deviationσ_(d) is also estimated.

The next step S8 consists in deciding whether this alarm corresponds toa bottom echo or indeed to a true echo at non zero radial velocity. Thevalues of the Doppler di and of the inherent Doppler dp and also thecorresponding rms deviations σ_(d) _(i) and σ_(d) _(p) are available.

Represented in FIG. 2 are the probability distributions of the measuredDoppler d_(m) for two hypotheses: H₀ for (stationary) bottom echo and H₁for assumed true Doppler echo d_(i). H₀ is centered on d_(p) with a rmsdeviation (σ_(d) _(p) ²+σ_(d) _(p) ²)^(1/2) and H₁ is centered on d_(i)with a rms deviation σ_(d) _(i) .

To decide, d_(i)−d_(p) is calculated and a threshold S is chosen: ifd_(i)−d_(p)>S, there is a true echo. The value of S is obtained on thebasis of the values of P_(f) which is the probability of decidingwrongly that a bottom echo is true.

The process of discrimination between true echo with non zero radialvelocity and bottom echo for each alarm detected by HFM is repeated.Next, among the HFM alarms detected and sorted, one undertakes theelimination S9 of the alarms which correspond to the bottom echoes (orto true echoes with zero radial velocity).

In step S10 is obtained an image of the tracks (series of alarms as afunction of time and direction) which is ridded of the false alarms andin particular the bottom echoes, all the better when they are strong andhence a nuisance.

1. A method of processing signals received corresponding to an emittedsignal, the emitted signal comprising by recurrence two pulses, a firstDoppler tolerant broadband pulse and a second Doppler intolerantbroadband pulse, said method comprising: detecting objects performed onthe part of the signal received corresponding to the first Dopplertolerant broadband pulse transmitted at each recurrence and providing analarm for each object detected; applying a matched filtering to thesignal received corresponding to the second Doppler intolerant broadbandpulse transmitted for each recurrence, said filtering making, for eachdetected object having satisfied at least a predetermined criterion ofselection, correlations between said signal and several dopplerizedcopies of said transmitted second pulse, said correlations producingseveral signals corresponding to several Doppler channels; andperforming a Doppler classification of the detected objects; whereinsaid Doppler classification is performed by comparing with a thresholdthe Doppler of every detected object having satisfied at least apredetermined criterion of selection, this Doppler being determined fromthe signals resulting from the application of the matched filtering. 2.The method of processing signals as claimed in claim 1, wherein thepredetermined criterion applied to every detected object is based on acomparison of its energy with a predetermined threshold.
 3. The methodof processing signals as claimed in claim 2, further comprising a stepof applying a first matched filtering to part of the signal receivedcorresponding to the Doppler tolerant pulses said applying step takingplace before the detection of objects and providing a signalcorresponding to an energy E_(HFM)(v,t).
 4. The method of processingsignals as claimed in claim 3, wherein said first matched filteringcomprises: correlating the part of the signal received corresponding toa Doppler tolerant pulse with a copy of the said Doppler tolerant pulse,applying rms detection to the correlated signal and providing signalsrepresenting the energy as a function of channel and time E_(HFM)(v,t).5. The method of processing signals as claimed in claim 1, wherein thestep of detecting objects comprises: searching for the local energymaxima E_(HFM)(v,t) by comparison with a predetermined energy thresholdE_(s), normalizing the maxima obtained by calculation for each localmaxima of the value (E_(HFM)−M)/σ, M being the mean of the referencenoise and σ the corresponding standard deviation, eliminating the maximaof lower normed energy, selecting the alarms corresponding tonon-eliminated normed maxima that are greater than a predeterminedthreshold of normed energy E_(SN).
 6. The method of processing signalsas claimed in claim 4, wherein said step of detecting objects comprises:searching for the local energy maxima E_(HFM)(v,t) by comparison with apredetermined energy threshold E_(s), normalizing the maxima obtained bycalculation for each local maxima of the value (E_(HFM)−M)/σ, M beingthe mean of the reference noise and σ the corresponding standarddeviation, eliminating the maxima of lower normed energy, selecting thealarms corresponding to non-eliminated normed maxima that are greaterthan a predetermined threshold of normed energy E_(SN).
 7. The method ofprocessing signals as claimed in claim 1, further comprising a step ofestimating Doppler d_(i) of the detected objects satisfying at least onepredetermined criterion, and/or its associated standard deviations σ_(d)_(i) , said estimation being made on the basis of the signals arisingfrom the Doppler channels.
 8. The method of processing signals asclaimed in claim 7, wherein the inherent Doppler is estimated at eachinstant: either on the basis of a Doppler of the part of the signalreceived corresponding to the reverberation of the Doppler intolerantpulses, or, on the basis of a reverberation spectrum obtained by anemitted frequency pulse (FP) code.
 9. A method of processing signalsreceived corresponding to a signal emitted comprising by recurrence twopulses, a first Doppler tolerant broadband pulse and a second Dopplerintolerant broadband pulse, said method comprising forming a firstchannel comprising the part of the signal received corresponding to theDoppler tolerant pulses, and a second channel comprising part of thesignal received corresponding to the Doppler intolerant pulses, applyinga first matched filtering to the signal on the first channel followed byan object detection process, providing an alarm for each objectdetected, selecting in the second channel objects satisfying at least apredetermined energy criterion, applying a second matched filtering onthe signal of the second channel around the selected objects, estimatingin the second channel the Doppler of the selected objects, estimatinginherent Doppler, classifying the selected objects by discriminationbetween bottom echoes and true echoes on the basis of the values of theDoppler of the selected objects estimated in the second channel and ofthe inherent Doppler, and eliminating in the first channel the alarmsdetected corresponding to bottom echoes.
 10. A method of processingsignals received corresponding to a signal emitted comprising byrecurrence two pulses, a first Doppler tolerant broadband pulse and asecond Doppler intolerant broadband pulse, said method comprising:forming a first channel comprising the part of the signal receivedcorresponding to the Doppler tolerant pulses, and a second channelcomprising part of the signal received corresponding to the Dopplerintolerant pulses; applying a first matched filtering to the signal onthe first channel followed by an object detection process; providing analarm for each object detected; selecting in the second channel objectsbased upon the provided alarms that satisfy at least a predeterminedenergy criterion; applying a second matched filtering on the signal ofthe second channel around the selected objects; estimating in the secondchannel a Doppler of the selected objects; estimating inherent Doppler;classifying the selected objects by discrimination between bottom echoesand true echoes on the basis of values of the Doppler of the selectedobjects estimated in the second channel and of the inherent Doppler; andeliminating in the first channel the alarms detected corresponding tobottom echoes; wherein the emitted signal come from a pulse of HFM type,and from a pulse of BPSK type, the two pulses being emitted in the samerecurrence.
 11. An active sonar comprising, means of emitting a signalcomprising by recurrence two pulses, a Doppler tolerant broadband pulseand a Doppler intolerant broadband pulse, and means of receiving thesignal emitted implementing the method of processing signals as claimedin claim
 10. 12. The active sonar as claimed in claim 11, wherein themeans of emission emit the two pulses at different instants with totallyor partly overlapping frequency bands.
 13. The active sonar as claimedin claim 11, wherein the means of emission emit the two pulsessimultaneously with distinct frequency bands.